Designed by Andre Herbemont and of generally similar
configuration to the Type 41, the Bleriot SPAD 51 had
fabric-covered metal wings and a wooden monocoque
fuselage, power being provided by a 420hp Gnome-
Rhone Jupiter nine-cylinder radial. The first prototype,
the Type 51-1, was flown on 16 June 1924, a second
modified prototype, the Type 51-2, following on 18
March 1925. Powered by a Jupiter IV and carrying an
armament of two 7.7mm MAC (Vickers) guns in the
upper wing, the Bleriot SPAD 51-2 was ordered for the
Polish air arm, the Lotnictwo Wojskowe, 50 being delivered
during 1925-26. A further prototype, the Type
51-3, with an improved propeller, flew on 7 September
1926, this offering a 12km/h speed superiority
and improved ceiling over the 51-2. On 30 August
1928, the final derivative of the basic model flew, this
being the Type 51-4 with a special 600hp version of the
Jupiter. Ten production Type 51-4s were completed,
one being sold to Turkey and another to the Soviet
Union, these having provision for two fuselage-mounted
Vickers and two wing-mounted Darne
machine-guns, and a 420hp Jupiter 9Ab.